CHAPTER IV 

 THE SUN'S GIPT OF HEAT 



The sun is not only the ruler of the solar system in that 

 it holds the planets in their orbits as they revolve about it ; 

 it also controls the activities upon the planets since it fur- 

 nishes them with their heat and light. Without the heat 

 of the sun the earth would be a cold, barren, lifeless, inert 

 ball of matter and nothing more. The sun's gift of heat is 

 all important. 



Everybody has observed many of the effects of heat. It 

 melts ice. It converts water into steam. It cooks food. 

 Thus we see that heat has the ability to cause change. The 

 capacity for causing change, for overcoming resistance, for 

 doing work, is called energy. Heat is therefore a form of 

 energy. 



A body may have through its position or its composition 

 the ability to do work without actually being at work. It 

 is then said to have potential energy. The moment a body 

 begins to do work, its energy is called kinetic energy. Either 

 kind of energy may be transformed into the other. 



A brick on a chimney top has potential energy owing to 

 its position. If some force pushes it off, its potential energy 

 is transformed into kinetic energy. When you wind a 

 clock, the energy you expend is transmitted to the spring, 

 and the spring is wound into such a position that it possesses 

 potential energy. Thus your kinetic energy is stored up 



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